The field of this invention relates to a method and apparatus for performing modulation of a radio frequency (RF) signal, and in particular to a method and apparatus for performing modulation of a radio frequency signal within a digital-to-RF converter.
The rapid advances in deep sub-micron CMOS (Complementary Metal-Oxide-Semiconductor) processes have led to digital-to-RF transmitter architectures that are becoming increasingly more popular due to their high efficiency. However, a key issue with the use of such digital-to-RF transmitter architectures is that the digital code-words used to control any analogue block within the digital-to-RF transmitter architecture introduce so-called DAC (Digital to Analogue Converter) images at the clock frequency of the digital code-word logic, and at harmonics thereof. Such DAC images may violate regulatory requirements, such as FCC (Federal Communications Commission) requirements, as well as certain in-device co-existence requirements such as PSD (Power Spectral Density) requirements, etc.
Such DAC images may be suppressed byway of a sinc function applied to the DAC sample-and-hold to ‘notch out’ the DAC images. However, the use of a sinc function is only good for narrow band systems, and is not appropriate for wide-bandwidth systems.
A more conventional approach to suppressing the DAC images is by way of additional analogue band-pass filtering, whereby the DAC images are attenuated by an analogue filter at the output of the digital-to-RF transmitter. However, such RF filtering incurs extra cost for the end device, and introduces pass-band loss into the transmit path. Such RF filters are particularly costly to implement for wide-bandwidth systems.
Thus, there is a need for an efficient and cost effective solution to the problem of DAC images within digital-to-RF transmitter architectures.